This invention relates to a resin encapsulation type semiconductor device, particularly to a semiconductor device encapsulated within an epoxy resin composition comprising an epoxy resin, a novolac resin curing agent and an organic tertiary phosphine compound.
Epoxy resins have widely been used as highly reliable electrical insulating materials for their excellent electrical properties, mechanical properties and humidity resistance for encapsulation of impregnation of semiconductor devices, electronic parts as well as electrical parts. In particular, most semiconductor devices, including integrated circuits, large scale integrated circuits, transistors and diodes are encapsulated using epoxy resin compositions for low pressure molding. The conventional epoxy resin compositions for electrical insulating materials have been most generally used by curing an epoxy resin with a curing agent such as a polyamine, an acid anhydride or a phenol resin together with a curing accelerator such as a tertiary amine or an imidazole. Particularly, in the field of encapsulation of semiconductor devices, there has been used an epoxy resin composition comprising an epoxy resin, a novolac resin curing agent, and an imidazole curing accelerator, which is the most excellent in various respects such as humidity resistance, reliability and moldability, as disclosed in U.S. Pat. No. 3,493,630. The semiconductor device encapsulated within such an epoxy resin composition can advantageously be produced at low cost and on a large scale, as compared with those encapsulated within metals or ceramics. On the other hand, it is inferior in humidity resistance and electrical properties at high temperatures. An encapsulated semiconductor device may sometimes be used or stored under conditions of high temperature and/or high humidity. When an epoxy resin encapsulation type semiconductor device is used at a high temperature and high humidity, electrical insulation of the semiconductor device may be lowered lowering its performance, for example, by increasing leak current or cause corrosion of aluminium electrodes or wiring, sometimes leading to failure or break down of wiring. Also, when it is used at a high temperature, there may be a deterioration of electrical properties of encapsulting resin, such as volume resistivity, which also results in an increase of leak current. In particular, semiconductor elements having a highly sensitive MOS structure on the surface or having a PN junction to which reverse bias is applied, suffers frequently from such inconveniences as increase leak current through channeling.
The inferior humidity resistance of an epoxy resin encapsulation type semiconductor device may be due to hygroscopicity or water permeability possessed by a cured epoxy resin on one hand, while it may also be due to hydrolyzable chlorine or ionic impurities contained in cured resin composition on the other. The latter hydrolyzable chlorine and ionic impurities inevitably remain because of the manufacturing technique employed. When moisture permeates into encapsulating resin, it may be associated with chlorine ions or other ionic impurities, whereby the insulating property of the device may suffer or corrosion of electrodes may result.
On the other hand, decrease of electrical properties at higher temperatures may be considered to be due to activated thermal motion of ionic impurities or polar substances contained in minute amounts in encapsulating resins. Particularly, when a voltage is applied on a semiconductor device, motion of ionic impurities or polar substances will be activated by the influence of electric field, whereby marked lowering in electrical properties may locally occur.
Especially under hot and humid conditions, there may occur complex reactions between humidity, ionic impurities, etc. and the action of the electric field, to thereby damage the characteristics of semiconductor devices, cause the device to deteriorate, and shorten its life.
In order to improve humidity resistance of an epoxy resin encapsulation type semiconductor device or to improve electrical characteristics thereof at higher temperatures, one can consider that hydrolyzable chlorine or ionic impurities should be removed. But it is practically impossible to remove these impurities completely for the reasons mentioned above.
For evaluation of humidity resistance of a resin encapsulation type semiconductor device, there may be employed a method such as the pressure cooker test (PCT), in which a device is exposed to a saturated water vapor at 85.degree. C. or 120.degree. C. for examination of the extent of deterioration of aluminum wiring through corrosion, or the bias-pressure cooker test (bias-PCT), in which a device on which voltage is applied is exposed to a hot saturated water vapor. On the other hand, for evaluation of electrical characteristics at high temperatures, there is generally employed the so called Bias-Temperature test (BT test), in which the quantity of leak current is examined by application of a bias voltage at a temperature from 80.degree. C. to 150.degree. C.